Weber 36 DCD "choke" vacuum leak
 

The choke or "Starter Plunger" according to the diagram is giving me a nasty vacuum leak on one of my rear carb. The other one is fine. What can cause this? If I put my thumb over the hole it idles smooth as can be. But soon after I take my thumb off is sputters and putters and I loose the rear rotor. The diagram isn't too clear what all is in that system. I don't see any kinda o-ring or anything that could be leaking.
Any help is appreciated.

(You'e looking at numbers 3,4,5,6,66,67,68)
http://www.piercemanifolds.com/36DCD.pdf

Pull those parts out and clean them, check the bore in the carb and clean that too. Sounds like something is sticking.